I think it is personal preference anymore. It really only matters on guns that have something you need to actuate on this side or that, i.e. M98 Mauser , M1903 Springfield, 1917 Enfield conversions or commercials based on those designs. The bolt release levers generally cause you to want to put the thumb screws opposite. Some ejection ports may make you want to put them opposite as well. Neither of those are issues with Win 70's.
